Optical fibers are key components in modern telecommunications systems. Optical fibers comprise strands of glass capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. In essence, an optical fiber is a small diameter waveguide characterized by a core with a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
Optical fiber Bragg gratings are important elements for selectively controlling specific wavelengths of light within an optical fiber. A typical Bragg grating comprises a length of optical fiber including a plurality of perturbations in the index of refraction substantially equally spaced along the fiber length. These perturbations selectively reflect light of wavelength .lambda. equal to twice the spacing .LAMBDA. between successive perturbations, i.e. .lambda.=2n.sub.eff .LAMBDA., where .lambda. is the vacuum wavelength and n.sub.eff is the effective refractive index of the propagating mode. Light of the selected wavelength .lambda. is reflected back to point of origin, and the remaining wavelengths pass essentially unimpeded. Such Bragg gratings are useful in a variety of applications including filtering, stabilizing semiconductor lasers, reflecting fiber amplifier pump energy, and compensating for fiber dispersion.
Bragg gratings in optical fibers are conveniently fabricated by providing a fiber having a core doped with one or more materials sensitive to ultraviolet light, such as a fiber having a core doped with germanium oxide, and then exposing the fiber at periodic intervals to high intensity ultraviolet light from an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce perturbations in the index of refraction. The appropriate periodic spacing of the perturbations to achieve a conventional grating can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams.
A difficulty with conventional fiber Bragg gratings is that they filter only a fixed wavelength. Each grating selectively reflects light in only a narrow bandwidth centered around .lambda.=2n.sub.eff .LAMBDA.. However, in many applications, such as multiplexing, it is desirable to have a grating whose wavelength response can be tuned, that is, controllably altered.
A further difficulty with fiber grating devices relates to the sensitivity of the fiber gratings to temperature changes. Temperature-compensating packages have been designed to address this drawback. See e.g. Morey et al., U.S. Pat. No. 5,042,898, issued Aug. 27, 1991, and Yoffe et al., "Passive Temperature Compensating Package for Optical Fiber Gratings," Applied Optics, Vol. 34, p. 6859, Oct. 20, 1995.
A tunable fiber grating has been attempted with use of a piezoelectric element to strain the grating. See Quetel et al., 1996 Technical Digest Series, Conf on Optical Fiber Communication, San Jose, Calif., Feb. 25-Mar. 1, 1996, Vol. 2, p. 120, paper No. WF6. However, the strain produced by piezoelectric actuation is relatively small, limiting the tuning range of the device. Moreover, piezoelectric activation requires a continuous application of relatively high voltage, e.g., approximately 100 volts for 1 nm strain.
The present invention provides a device and method for tuning optical fiber gratings involving the use of thermally-induced strain on the fiber. The device has an enhanced tuning range and does not require continuous power, as compared with previous devices relying upon piezoelectric elements. Additionally, temperature controls are incorporated into the device, hence, the sensitivity of the grating to temperature changes can be incorporated into the programming of the wavelength tuning, avoiding the need for temperature-compensating packages.